Non-electrification polymeric composite material

ABSTRACT

A non-electrification polymeric composite material composed of 60-95% by weight of a copolymer of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA), 1-10% by weight of electrically conductive carbon, and 5-20% by weight of carbonaceous fiber powder. The weight ratio of the electrically conductive carbon contained in the polymeric composite material to the carbonaceous fiber powder also contained in the material in from 1/9 to 7/4, preferably from 2/8 to 5/5. The polymeric composite material is especially suitable as a semiconductor holder, particularly for semiconductor wafers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-electrification polymericcomposite material, more particularly to a non-electrification polymericcomposite material capable of being suitably employed to hold asemiconductor substrate.

2. Description of the Prior Art

In producing a semiconductor device, there has been generally taken aseries of such steps as etching a semiconductor wafer, washing thewafer, and the like, with the semiconductor wafer being held by aholder, or the like. Heretofore, as a material of this holder forholding the semiconductor wafer, has been used a chemically resistantand heat-resisting fluoro resin such as polytetrafluoroethylene(referred to as PTFE), a copolymer of perfluoro(alkyl vinyl ether) andtetrafluoroethylene (referred to as PFA), or the like. Each of suchpolytetrafluoroethylene (PTFE) and copolymer of perfluoro(alkyl vinylether) and tetrafluoroethylene (PFA) is excellent in electric insulationand has as high an electric resistivity as 10¹⁸ -10¹⁹ Ω.cm at roomtemperature, and hence are apt to be readily electrified due tofriction. Accordingly, in the case where the holder made of such fluororesin material is dried by utilization of the centrifugal force and whenrotated at a high speed, the holder is charged with static electricitydue to friction between this holder and air. Consequently, the adjacentdust, dirt or the like is attracted to the thus electrified holder andthen sticks to the surface of the semiconductor wafer, thereby resultingin decrease in yield of semiconductor chips.

One technique that overcomes the foregoing problem is described inJapanese Patent Application No. Tokukaisho 58-207651. This prior artdiscloses that mixing an electric conductor such as carbonaceous fiber,carbon black, or the like with a fluoro resin such as a copolymer oftetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA) produces acomposite material which does not lose its chemical and heat resistancein respect to the fluoro resin contained therein and at the same timeexhibits non-electrification characteristic per se. However, this priorart does not suggest, for example, any particular ratio of thecarbonaceous fiber mixed with the copolymer of tetrafluoroethylene andperfluoro(alkyl vinyl ether) (PFA) to this copolymer. When, for example,carbon is added to the fluoro resin to make up a composite material,indiscreetly based on the disclosure of this prior art, the mechanicalstrength of the fluoro resin is affected by the so added carbon andfurther the melt viscosity of the resin is enhanced. In consequence, itbecomes difficult to treat the composite material injection molding,owing to the higher melt viscosity thereof. In the etching process,furthermore, the carbon contained in the composite material is etchedoff into the etching solution, thereby presenting such a problem thatthe semiconductor wafer is soiled by the thus dropped carbon.Accordingly, the material merely deprived of its frictionalelectrification properties still does not sufficiently serve as thematerial of the holder employed for the production of the semiconductordevice to hold the semiconductor wafer.

SUMMARY OF THE INVENTION

With a view to solving the foregoing problems, a primary object of thepresent invention is to provide a novel and improved non-electrificationpolymeric composite material.

A further object of the invention is to provide a non-electrificationpolymeric composite material which does not lose its chemical and heatresistance in respect to the polymeric material contained therein and atthe same time has a nonelectrification characteristics.

In order to accomplish the above objects, a nonelectrification polymericcomposite material according to the invention comprises 60-95% by weightof a polymeric material, 1-10% by weight of electrically conductivecarbon, and 5-20% by weight of carbonaceous fiber powder.

In a preferred embodiment, said polymeric material is a homopolymer oftetrafluoroethylene.

In another preferred embodiment, said polymeric material is a copolymerof tetrafluoroethylene and another copolymerizable monomer.

In still another preferred embodiment, said copolymer is a copolymer oftetrafluoroethylene and one copolymerizable monomer selected from thegroup consisting of hexafluoropropylene, ethylene, vinylidene fluoride,trifluoroethylene, and perfluoro(alkyl vinyl ether).

In a further preferred embodiment, the weight ratio of said electricallyconductive carbon to said carbonaceous fiber powder is from 1/9 to 7/4.

In a still further preferred embodiment, the weight ratio of saidelectrically conductive carbon to said carbonaceous fiber powder is from2/8 to 5/5.

In a yet further preferred embodiment, said carbonaceous fiber powder isformed of particles each having a diameter of 3-30 μm and an averagelength of 10-10,000 μm.

According to the invention, by mixing 1-10% by weight of electricallyconductive carbon and 5-20% by weight of carbonaceous fiber powder with60-95% by weight of a polymeric material, and determining the weightratio of the electrically conductive carbon to the carbonaceous fiberpowder to be from 1/9 to 7/4, particularly from 2/8 to 5/5, there isprepared a non-electrification composite material which does not losesuch characteristics as the chemical and heat resistance for thepolymeric material contained therein and whose carbon component isremarkably prevented from dropping off from the composite material to asgreat an extent as possible, thereby to achieve an excellentmoldability.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the invention will be apparent from thefollowing description taken in connection with the accompanying drawingwherein:

The FIGURE in the drawing is a perspective view showing an embodiment ofa holder, for holding a semiconductor wafer, made of anon-electrification polymeric composite material according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawing, preferred embodiments of the invention aredescribed below.

The FIGURE in the drawing is a perspective view illustrating anembodiment of a holder 2 made of a non-electrification polymericcomposite material according to the invention. This holder 2 which holdsa plurality of semiconductor wafers 1 is in the shape of the letter H inthe section perpendicular to its axial line, and comprises a pair ofwalls 3,4 facing each other for holding the peripheral edges of thesemi-conductor wafers 1 and a pair of connecting portions 5 and 6 forconnecting the walls 3 and 4. In the walls 3 and 4, there are formed apair of sets of grooves 7 and 8, in which the semiconductor wafers 1 arerespectively fitted. As a material of the holder 2, is used thenon-electrification polymeric composite material according to theinvention. A series of such steps as etching, washing, and drying thesemiconductor wafers 1 is taken, with the wafers 1 being held by theholder 2.

As the polymeric material to be employed in the invention, a homopolymerof tetrafluoroethylene or a copolymer of tetrafluoroethylene and anothercopolymerizable monomer is preferred. Such a copolymer includes acopolymer of tetrafluoroethylene and one copolymerizable monomerselected from the group consisting of hexafluoropropylene, ethylene,vinylidene fluoride, trifluoroethylene, and perfluoro(alkyl vinylether). The copolymer of tetrafluoroethylene and perfluoro(alkyl vinylether) (referred to as PFA) is particularly preferable as thiscopolymer. As the electrically conductive carbon to be mixed with thepolymeric material there is preferred an organic material having a highelectric conductivity, for example, carbon black. As the carbonaceousfiber powder to be likewise mixed with the polymeric material, there ispreferred one which is formed of finely powdered carbonaceous fiberwhose each particle has a diameter of approximately 3-30 μm and anaverage length of approximately 10-10,000 μm.

It is preferred that the amount of the electrically conductive carboncontained in the non-electrification polymeric composite material bedetermined within the range of from 1 to 10% by weight based on thetotal weight of the composite material, while it is preferred that theamount of carbonaceous fiber powder contained in the composite materialbe determined within the range of from 5 to 20% by weight based on thetotal weight of the composite material. When the content of theelectrically conductive carbon is more than 10% by weight, the strengthof the polymeric material is affected by electrically conductive carbonmixed therewith in such amount and further the melt viscosity of thematerial is enhanced. Hence it becomes difficult to subject thecomposite material to injection molding owing to its higher meltviscosity. On the contrary, when the content of the electricallyconductive carbon is less than 1% by weight, the composite material isobserved to remarkably exhibit frictional electrificationcharacteristics. In the meantime, when the content of the carbonaceousfiber powder is more than 20% by weight, the carbon contained in thecomposite material drops off and is dissolved in the etching solutionduring the etching process of etching the semiconductor wafer, thusleading to soiling of the wafer. In contrast, when the content of thecarbonaceous fiber powder is less than 5% by weight, the processabilityof the composite material is deteriorated.

Furthermore, it is preferred that the weight ratio of the electricallyconductive carbon to the carbonaceous fiber powder be from 1/9 to 7/4,particularly from 2/8 to 5/5. When the weight ratio of the electricallyconductive carbon to the carbonaceous fiber powder is below 1/9, thecomposite material is observed to remarkably exhibit frictionalelecrification characteristics. On the contrary, when the weight ratioof the electrically conductive carbon to the carbonaceous fiber powderis above 7/4, the processability of the composite material isdeteriorated. Meanwhile, when the weight ratio of the electricallyconductive carbon to the carbonaceous fiber powder is from 2/8 to 5/5,the composite material substantially loses the frictionalelectrification property and obtains the excellent processability.

Accordingly, in view of the foregoing, the contents of the respectivecomponents to make up the non-electrification polymeric compositematerial acording to the invention are to be determined as definedabove.

The invention will be explained in more detail by the followingexamples.

EXAMPLES 1-3

A total of 10 parts by weight of (a) electrically conductive carbon,Ketjen black EC (trade name, manufactured by Lion-Agnes Co.) and (b)carbonaceous fiber powder, BESFIGHT HTA 3000 (trade name, manufacturedby Toho Rayon Co., Ltd.) in the various weight ratios as shown in Table1 below were mixed with 90 parts by weight of a copolymer oftetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA), NeoflonPFAAP-210 (trade name, manufactured by Daikin Industries, Ltd.) which isexcellent in injection-moldability, by means of a kneader heated at 350°C., for approximately 5-20 minutes to obtain polymeric compositematerials. The thus obtained polymeric composite materials were thenheat-pressed at 350° C. to be molded into sheet-shaped samples eachhaving a thickness of 1 mm. Thereafter the samples thus molded weresubjected to tests of the frictional electrification properties, thedegree of the dropping-off of the carbon, and the processability.

The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                               Electrically                                                                  Conductive                                                                    Carbon/     Frictional                                                        Carbonaceous                                                                              Electrifi-                                                                              Degree of                                        Example                                                                              Fiber Powder                                                                              cation    Dropping-off                                                                           Process-                                No.    (weight ratio)                                                                            Property  of Carbon                                                                              ability                                 ______________________________________                                        1      2/8         A,˜B,˜C                                                                     D˜E                                                                              G                                       2      3.5/6.5     C         E˜D                                                                              G                                       3      5/5         C         E˜D                                                                              I                                       ______________________________________                                    

The procedures of these tests and the methods of illustrating theresults are as follows.

Test of Frictional Electrification

The sample which were composed of the copolymers of tetrafluoroethyleneand perfluoro(alkyl vinyl ether) (PFA), and electrically conductivecarbon and carbonaceous fiber powder both mixed with the copolymers insuch weight ratios as shown in Table 1 were rubbed with nylon or clothso as to be electrified. Then, attempts were made to attach a sheet ofthick typing paper having a size of 2×2 cm with the thus electrifiedsamples.

The results of such attemps are shown by the following symbols A, B, andC in Table 1:

A: the sheet of typing paper was attached with the samples.

B: the sheet of typing paper was attached with the samples, and droppedfrom the samples by its own weight when the samples were turnedupside-down.

C: the sheet of typing paper was not attached with the sampleswhatsoever.

Test of the Degree of Dropping-off of Carbon

Filter paper was placed on the surfaces of the samples. Then, thesamples were rubbed with filter paper while applying force to filterpaper with a finger, thereby examining to what degree the black ofcarbon contained in the samples were adheringly transferred to thefilter paper. Thus the degree of dropping-off of the carbon from thesamples was tested. The results are shown by the following symbols D, E,and F in Table 1:

D: the black of carbon contained in the samples was not transferred tofilter paper.

E: the black of carbon contained in the samples was somewhat adheringlytransferred to filter paper.

F: the black of carbon contained in the samples was so apparentlyadhereingly transferred to filter paper as to be ascertained by thenaked eye.

This test was intended to examine the removability of carbon from thesamples containing it. There is no such possibility that the compositematerials estimated at D or E will soil the etching solution even whenthey are employed for the semiconductor producing apparatus.

Test of Processability

The polymeric composite materials according to the invention werepractically treated by the injection molding, thus testing theirprocessability. Next, comparisons were made between theinjection-moldability of the polymeric composite materials according tothe invention and that of the polymeric material merely composed of thecopolymer of tetrafluoroethylene and perfluor(alkyl vinyl ether) (PFA).The results are shown by the following symbols G, H, and I in Table 1:

G: the injection-moldability of the polymeric composite material of theinvention was substantially the same as that of the polymeric materialonly composed of the copolymer.

H: the viscosity of the polymeric composite material of the inventionwas substantially higher than that of the polymeric material onlycomposed of the copolymer and hence it was difficult to make a treatmentof the injection molding for the former.

I: the injection-moldability of the polymeric composite material of theinvention was judged to be in the intermediate state between G and H.

EXAMPLES 4 AND 5

Non-electrification polymeric materials were prepared in the same manneras in examples 1-3 except that a total of 15 parts by weight of (a)electrically conductive carbon and (b) carbonaceous fiber powder in suchweight ratios as shown in Table 2 below were mixed with 85 parts byweight of the copolymers of perfluoro(alkyl vinyl ether) andtetrafluoroethylene (PFA). The thus prepared polymeric compositematerials were then molded into samples to be tested. Thereafter, thethus molded samples were subjected to the tests of the frictionalelectrification property, the degree of dropping-off of carbon, and theprocessability, in the same manner as described in Examples 1-3.

The results are shown in Table 2. The procedures of these tests and themethod of representing the results are the same as those of Examples1-3.

                  TABLE 2                                                         ______________________________________                                               Electrically                                                                  Conductive                                                                    Carbon      Frictional                                                        /Carbonaceous                                                                             Electrifi-                                                                              Degree of                                        Example                                                                              Fiber Powder                                                                              cation    Dropping-off                                                                           Process-                                No.    (weight ratio)                                                                            Property  of Carbon                                                                              ability                                 ______________________________________                                        4       2/13       C         E˜D                                                                              G                                       5       3.5/11.5   C         E        G                                       ______________________________________                                    

REFERENCE EXAMPLES 1-5

Polymeric composite materials were obtained in the same manner as inExamples 1-3 except by mixing 4.8-20 parts by weight of onlycarbonaceous fiber powder with 80-95.2 parts by weight of the copolymersof perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA). Then thethus obtained composite materials were similarly molded into samples tobe tested. The samples were thereafter subjected to the tests of thefrictional electrification property, the degree of dropping-off ofcarbon, and the processability in the same manner as described inExamples 1-3.

The results are shown in Table 3. The procedures of the tests and themethod of illustrating the results are the same as those of Examples1-3.

                  TABLE 3                                                         ______________________________________                                                Content of                                                                    Carbonaceous                                                                             Frictional                                                 Reference                                                                             Fiber Powder                                                                             Electrifi-                                                                              Degree of                                        Example (parts by  cation    Dropping-off                                                                           Process-                                No.     weight)    Property  of Carbon                                                                              ability                                 ______________________________________                                        1       4.8        A         D        G                                       2       7.3        A         D        G                                       3       10         A         D        G                                       4       15         A         D        G                                       5       20         A         D        G                                       ______________________________________                                    

REFERENCE EXAMPLES 6-9

Polymeric composite materials were obtained in the same manner as inExamples 1-3 except by mixing 2-12 parts by weight of only electricallyconductive carbon with 88-98 parts by weight of the copolymers ofperfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA). Then thethus obtained composite materials were similarly molded into samples tobe tested. The samples were thereafter subjected to the tests in thesame manner as described in Examples 1-3.

The results are shown in Table 4. The procedures of the tests and themethod of illustrating the results are the same as those of Examples1-3.

                  TABLE 4                                                         ______________________________________                                                Content of                                                                    Electrically                                                                  Conductive Frictional                                                 Reference                                                                             Carbon     Electrifi-                                                                              Degree of                                        Example (parts by  cation    Dropping-off                                                                           Process-                                No.     weight)    Property  of Carbon                                                                              ability                                 ______________________________________                                        6        2         A         D        G                                       7        5         A         E˜D                                                                              G˜I                               8       10         C         F        I                                       9       12         C         F        H                                       ______________________________________                                    

REFERENCE EXAMPLES 10 AND 11

Polymeric composite materials were prepared in the same manner as inExamples 1-3 except by mixing a total of 14-15 parts by weight of (a)electrically conductive carbon and (b) carbonaceous fiber powder in suchweight ratios as shown in Table 5 below with 85-86 parts by weight ofthe copolymers of perfluoro(alkyl vinyl ether) and tetrafluoroethylene(PFA). Then the composite materials were similarly molded into samplesto be tested. These samples were thereafter subjected to the tests inthe same manner as described in Examples 1-3.

The results are shown in Table 5. The procedures of the tests and themethod of illustrating the results are the same as those of Examples1-3.

                  TABLE 5                                                         ______________________________________                                                Electrically                                                                  Conductive                                                                    Carbon      Frictional                                                                              Degree of                                       Reference                                                                             /Carbonaceous                                                                             Electrifi-                                                                              Dropping-                                       Example Fiber Powder                                                                              cation    off of  Process-                                No.     (weight ratio)                                                                            Property  Carbon  ability                                 ______________________________________                                        10      7/7         C         E       H                                       11       5/10       C         F       I                                       ______________________________________                                    

As apparent from Tables 3 and 4, the polymeric composite materials whichwere prepared by mixing either one of carbonaceous fiber powder andelectrically conductive carbon with the copolymers of perfluoro(alkylvinyl ether) and tetrafluoroethylene (PFA) do not meet all the desirablerequirements in regard to the frictional electrification property, thedegree of dropping-off of carbon, and the processability. Furthermore,as apparent from Table 5, the larger amount of electrically conductivecarbon is contained in the polymeric composite material, the lesspreferable the composite material becomes as an available material inview of the degree of dropping-off of carbon and the processability.

In contrast, as apparent from Tables 1 and 2, by mixing 1-10% by weightof electrically conductive carbon and 5-20% by weight of carbonaceousfiber powder with 60-95% by weight of the copolymer of perfluoro(alkylvinyl ether) and tetrafluoroethylene (PFA) in accordance with theinvention, there can be effected non-electrification polymeric compositematerials which do not lose the chemical and heat resistances of thecopolymers of perfluoro(alkyl vinyl ether) and tetrafluoroethylene (PFA)contained therein and which at the same time are adapted not to soil anyetching solutions during the etching process for the semiconductorwafers and further have excellent injection-moldability.

The non-electrification polymeric composite material according to theinvention is not restricted to the use of the material of the holder forholding the semiconductor wafer as mentioned above, and can be widelyapplied in the other various technical fields.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A non-electrification composite materialcomprising 60-95% by weight of a polymeric material selected from thegroup consisting of a homopolymer of tetrafluoroethylene and a copolymerof tetrafluoroethylene and one copolymerizable monomer selected from thegroup consisting of hexafluoropropylene, ethylene, vinylidene fluoride,trifluoroethylene, and perfluoro(alkyl vinyl ether), 1-10% by weight ofelectrically conductive carbon, and 5-20% by weight of carbonaceousfiber powder.
 2. A non-electrification composite material as claimed inclaim 1, wherein said polymeric material is a homopolymer oftetrafluoroethylene.
 3. A non-electrification composite material asclaimed in claim 1, wherein said polymeric material is a copolymer oftetrafluoroethylene and one copolymerizable monomer selected from thegroup consisting of hexafluoropropylene, ethylene, vinylidene fluoride,trifluoroethylene, and perfluoro(alkyl vinyl ether).
 4. Anon-electrification composite material as claimed in claim 1, whereinthe weight ratio of said electrically conductive carbon to saidcarbonaceous fiber powder is from 1/9 to 7/4.
 5. The non-electrificationcomposite material as claimed in claim 1, wherein the weight ratio ofsaid electrically conductive carbon to said carbonaceous fiber powder isfrom 2/8 to 5/5.
 6. A non-electrification composite material as claimedin claim 1, wherein said carbonaceous fiber powder is formed ofparticles each having a diameter of 3-30 μm and an average length of10-10,000 μm.